Washington's Martin Erat is helped off the ice after being hit from behind. / Joel Auerbach, Getty Images

by Mike Brehm, USA TODAY Sports

by Mike Brehm, USA TODAY Sports

Martin Erat, the Washington Capitals' key acquisition at the trade deadline, had to leave Saturday's game after a hit from behind.

Florida Panthers defenseman Erik Gudbranson was ejected for boarding for the hit.

Era'ts legs were splayed when he hit the boards. He was in obvious pain and had to be helped off the ice.

Washington captain Alex Ovechkin scored the first of his three goals during the five-minute power play.

Coach Kevin Dineen forgot to put a player in the penalty box to serve Gudbranson's penalty, so the Panthers remained short-handed after the penalty expired. The Panthers took another penalty and the Capitals scored on the carry-over in the second period.

"It was a bad coaching error by me," Dineen said.

After the game, Capitals coach Adam Oates called the hit a "pretty dangerous play."

No hearing for Gudbranson was announced on Sunday.

Erat is day-to-day and sat out Sunday night's game against the Tampa Bay Lightning.

"Marty felt way better than he anticipated," Oates said, "so I don't think it's going to be too long."

The Capitals gave up top prospect Filip Forsberg to land Erat, who has two years left on his contract.